The present invention relates to a procedure for generating the control signals in a push button box designed e.g. for the control of a crane, said push button box comprising at least one push button element, and to a push button box designed for implementing the procedure.
For generating two-step, multi-step or stepless control signals, there is currently no light-weight push button box of absolutely air-and-watertight construction that would also be suited for one-hand control. Such a control box is needed e.g. for an overhead travelling crane provided with a hanging push button box for stepless or stepwise motion control.
In certain push button boxes in current use, two-step or multi-step control is based on the utilization of the time differences between the contact actions of contact elements located at different levels in the direction of depression. In such a solution, contact elements located at different levels produce an output signal at different instants depending on the depth of depression of the button. There are also control boxes that use a handle mounted on a shaft passing through the box and, when the handle is turned, actuating a contact element with several contacts in the box in a stepwise manner, functioning much like a camshaft. The shaft may also be used to drive a signal source, such as a potentiometer, which supplies a stepless or a stepwise signal that is dependent on the angle of displacement of the control handle.
In special environments, e.g. wet spaces or spaces containing explosive substances, where the push button box has to be well sealed, the sealing increases the cost because every hole made in the box shell for the control gear has to be separately sealed in accordance with the environmental classification of the locality in question.
The motion required for the depression of single-step push buttons can nowadays be transmitted through a membrane or a flexible covering on the contact elements, thus rendering the enclosure of the push button box sufficiently tight and obviating the need for making and sealing a hole. There are also push buttons which use the Hall effect to produce 0-1 signals. Employing this principle makes it possible to achieve a good tightness of the box enclosure, because the push button incorporates no moving parts. Further, DE-patent publication 3008561 proposes a push button construction based on the bistable nature of a magnetic field. However, these principles cannot be reasonably applied to construct push buttons with multi-step or stepless control signal functions for use in enclosed control boxes.
A normal requirement concerning a push button used for stepless or stepwise control of crane movements is that it should give the user a good feel of the position of the operating arm employed. This feature is difficult to implement with currently used push button constructions because the feel of position in these constructions depends on several factors, e.g. possible bearing suspension of the operating arm and of the shaft going through the control box, counter forces presented by the contact element, etc.
In the control box constructions currently used, the relatively large size of the contact elements for stepwise control or of the signal transducer for stepless control, the shafts going through the box enclosure and the sealing arrangements needed to ensure a tightness corresponding to the environmental conditions, and the devices needed to produce the required feel of step are all factors increasing the weight of the control box. Besides being heavy, such a control box is ill adapted for one-hand operation.
The object of the present invention is to eliminate the drawbacks referred to above and to achieve a push button control box construction that provides an improved tightness of enclosure and a better feel of operating arm position and is also light enough to enable the control box to be used without difficulty with one hand only.